1. Field of the Invention
The present invention relates to a method for charging a lead acid battery while reducing a charging current stepwise, and an apparatus therefor.
2. Description of the Background Art
Two-stage constant current charging permitting short-time charging may be performed for charging a lead acid battery used for the purpose of cycle service for electric cars, carriages, golf carts, etc.
The two-stage constant-current charging is a method in which charging is performed with a large current before the voltage of the lead acid battery reaches a predetermined change-over voltage, and the moment the voltage reaches the change-over voltage, the charging is changed over to small-current charging and the small-current charging is continued for a predetermined time until the charging rate reaches a predetermined value, for example, 120% of the discharging rate.
In such a type charging method, the period of primary charging performed with a large current needed to be carried out as long as possible in order to shorten the charging time. Therefore, the change-over voltage needed to be set to be high. However, if the change-over voltage was set to be high, the charging method became uncontrollable to bring overcharging when there was such a situation that the voltage of the lead acid battery never reached the change-over voltage though the primary charging was continued. Therefore, the change-over could not but be set to be sufficiently low to leave a margin, so that the primary charging time was shortened undesirably. Accordingly, the quantity of electricity by the primary charging was reduced. In the background-art charging method, there was a problem that reduction of the charging time could not be attained sufficiently because the reduction of electricity was compensated for in the secondary charging.
On the other hand, a valve-regulated lead acid battery having no need for water supply has been popularized in recent years. In the valve-regulated lead acid battery, battery closure is completed by a method called negative electrode absorption type in which an oxygen gas produced at a positive electrode during charging is absorbed by reaction with a charged active material (Pb) in a negative electrode to thereby suppress production of a hydrogen gas from the negative electrode. A Pb-Ca alloy grid is generally used as a positive electrode grid of such a valve-regulated lead acid battery. In such a valve-regulated lead acid battery, however, there is a disadvantage that the life of the lead acid battery is shortened if the lead acid battery is used in an extreme condition such as high temperature, deep discharge, or the like. This is because a passivation layer of PbSO.sub.4 is formed in an interface between the grid and the active material to thereby cause a phenomenon of premature capacity loss (hereinafter referred to as PCL) that capacity is lowered prematurely. Since this PCL phenomenon is peculiar to the battery using a Pb-Ca alloy grid in a positive electrode, it may be conceived that a Pb-Sb alloy grid generally used in a flooded type battery can be applied also to the valve-regulated lead acid battery.
The battery using such a Pb-Sb alloy grid, however, has a characteristic that the rising of the battery voltage with the advance of charging becomes gentler and gentler as the charging and discharging is repeated. Hence, in the aforementioned charging method of performing controlling by detecting the change-over voltage, the battery voltage cannot reach the preset change-over voltage even if the charging advances so that the battery falls into an uncontrollable state. The uncontrollable state causes overcharging, and H.sub.2 O in the electrolyte is electrolyzed. In the valve-regulated lead acid battery, however, if water is consumed, the electrolyte is dried up prematurely to make it impossible to use the battery because the battery contains only a minimum amount of the electrolyte. Hence, there is the existing situation that the valve-regulated lead acid battery using a Pb-Sb alloy grid free from the PCL phenomenon as a positive electrode grid has been never put into practical use actually because the charging apparatus has been a barrier.